1. Field of the Invention
The present invention relates to the control of vapor generators and other devices of similar character. More specifically, this invention is directed to a digital control system for enhancing the availability of and efficiency and safety of operation of fossil fuel fired steam generators. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
The general state of the art with respect to automatic firing controls for coal burning furnaces is represented by the disclosure of U.S. Pat. No. 3,395,657 issued to J. A. Schuss on Aug. 6, 1968 and assigned to the assignee of the present invention; said U.S. Pat. No. 3,395,657 being incorporated herein by reference. The system of U.S. Pat. No. 3,395,657, which may be generally described as a "hard wired control", constituted a substantial step forward in the art when compared to the previous practice of operator exercised control over furnace start-up and shut-down.
It has been proposed to implement the control technique of U.S. Pat. No. 3,395,657 through use of a large general purpose plant computer which, in addition to burner start-up and shut-down supervision, monitors and controls numerous other functions within a power station; the furnace to be controlled typically providing steam for the driving of turbines connected to electrical generators.
Controls of the "hard wired" type have imposed a number of undesirable limitations on the designer and user. Thus, by way of example, all "hard wired" systems must be manually fabricated and installed and component damage resulting from wiring mistakes has been unavoidable. Further, shop simulation and field maintenance of "hard wired" systems is inefficient and expensive due, in part, to the need to follow a substantial number of wiring diagrams which are themselves generated at a rather high cost. A closely allied problem is the need to update all of these wiring diagrams after each logic modification or correction found necessary during simulation or field testing. Perhaps the major disadvantage of "hard wired" control systems, however, resides in the substantial difficulty of modification of the control once installed; i.e., "hard wired" controls are for all practical purposes inflexible. As a further significant disadvantage, increases in reliability realizable through the use of redundant circuits can be achieved in a "hard wired" control only at a comparatively high cost and the incorporation of means for self-checking the control systems and its components is thus both expensive and exceedingly difficult to implement.
The use of a plant installed general purpose computer for specialized control sub-loops such as coal mill; i.e., pulverizer; start-up and shut-down also possesses serious disadvantages. Bearing in mind that so-called "nuisance shutdowns" are extremely expensive to an electrical utility, use of a single large general purpose computer, perhaps with an equally expensive back-up computer, poses obvious functional and economic disadvantages. Thus, by way of example only, a failure in the computer or the power supply thereto totally unrelated to the apparatus being controlled would result in a "nuisance shutdown" in a system wherein the plant general purpose computer is utilized to control specialized ancillary equipment such as coal mills.
To summarize, the art has long needed a flexible and reliable method for exercising control over specialized operations which form part of the overall control procedure for coal fired furnaces of steam generators. The principal attributes of the desired control system are simplicity, ease of installation, self-checking capability, ability to directly interface with existing computer equipment to facilitate the monitoring of control system performance, minimum expense commensurate with successful and safe operation, virtual elimination of "nuisance shutdowns" and enhancement of plant availability.